This invention relates to a converter circuit arrangement for transferring power from an electrical two-conductor current loop to a functional unit. The circuit comprises an inductive coupling (transformer) and an AC/DC converter based on the switch mode. More specifically this is here derived from a DC/DC converter of the "Boost Mode" type. The current loop conducts an alternating current of a fixed frequency and amplitude for the power transfer, and signals to and from the functional unit are superimposed on the power transfer current in the current loop.
In the inductive coupling the current loop constitutes a primary winding and a rectifier device is located at the secondary side for DC supply to the functional unit. Moreover there is included a voltage regulator for controlling an electronic switch for operation in switch mode as mentioned, a power coil and at least one capacitor.
The invention has been primarily developed in connection with a so-called fieldbus for interconnecting several functional units, as for example measuring instruments or control units in process control installations, and the like. For example, a current loop bus of the type Current Bus Mode according to IEC 1158-2, can be of interest here. This fieldbus or current loop constitutes a two-conductor line, where functional units connected to it communicate, by means of signals, through this line, at the same time as they are supplied with required power in the form of current supply through the same pair of conductors. In such a current loop bus the functional units are connected in series in the bus. Power for current supply from the bus is tapped over the equivalent impedance introduced by the functional unit concerned, into the two-conductor line. This is fed with a constant alternating current from a central unit. Each of the functional units appears as a series load in the current loop bus, and draws power being proportional to the real part of the series load. In practice the connection of functional units to the current loop is effected inductively by the bus' forming one or more windings at the primary side of a transformer. With such an inductive connection the secondary side of this transformer can be an integrated part of the functional unit concerned.
In addition to the above mentioned power or current supply for the operation of associated functional units, the inductive coupling can also be adapted to provide for signal transfer or communication between the fieldbus and the respective functional units. Such signals are superimposed on the power feed, and in practice will normally have a signal strength lying several decades lower than the current amplitude for power transfer. Accordingly it is very important that the power signal has a sinusoidal shape as closely as possible, so that required filtering at the individual functional units will be simplified. Moreover the load represented by each unit with respect to the current loop bus, should be as linear as possible in order to prevent the functional units from introducing harmonic noise into the fieldbus network. Such noise will be disturbing and detrimental for said signal communication. Harmonic noise components from the power supply part or from non-linearities in the individual functional unit, will be spread into the signal band for communication and thereby reduce the signal-to-noise ratio.
These considerations also apply in the alternative of providing for signal transfer to/from the fieldbus by a separate inductive coupling or transformer.
In fieldbus systems or installations of actual interest to this invention, the feed frequency of the AC supply is substantially higher than the common mains frequency, namely 16 kHz in a specific fieldbus embodiment. The frequencies employed for the above discussed communication or signal transfer, usually are significantly higher than this current supply frequency. As will further appear from the description below, the converter circuits for the functional units comprise an electronic switch, such as a transistor switch, adapted to be controlled at a fixed rate or repetition frequency, which typically can be about 350 kHz. The examples of typical frequencies given here form a part of the background for the particular solutions to be described in the following description.
It is desirable that the voltage drop over each connected functional unit for its current supply, is as low as possible and preferrably an ohmic (resistive) load. Possible imaginary voltage drops occurring, will mean that the feed voltage for the AC supply of the current loop will be unnecessarily high, and the total efficiency of the power or the current supply to the complete fieldbus network will be lower.
With respect to voltage regulators there are various types of known and described configurations, including such which are based on switch mode, with an active breaking of applied current and subsequent transfer of the electric energy from an inductive component (power coil) to the load. In the circuit solution being described in the following, there is employed a particular power circuit configuration for the regulator, being known as a "Boost Mode" configuration (booster). This particular form and related forms of voltage regulator configurations are more closely described for example in the publication "High power factor preregulators for off-line power supplies" by Lloyd H. Dixon, Jr. Unitrode Corporation, published in Unitrode Power Supply Design Seminar 1990. This regulator configuration is intended for use in so-called DC/DC converters where a rectified voltage is converted to another rectified voltage. In the circuit solution concerned, a rectifier function is combined with a regulator function and a "Boost Mode" power circuit is employed in order to implement a conversion from AC to DC.
Other examples of rectifier arrangements which, at least superficially, can be considered to have features resembling parts of the rectifier circuits of interest to this invention, are to be found in Swedish patent publication 452.226 and European patent publication 0387.735. These two examples, however, do not relate to current-fed networks of fieldbus type as referred to above; nor are these known rectifiers designed in consideration of other particular conditions and environments that are associated with the fieldbus or current loop networks, being of interest here. SE 452.226 shows a resonant circuit as a filter at the input of a rectifier circuit, the purpose of this filtering being to apply a specific wave shape to the rectifier bridge. The European patent publication referred to also describes use of a series resonant circuit as a filter, for the purpose of delimiting a reverse current (reverse recovery) from the rectifier circuit.
Moreover, short mention shall here be made of three US patents having a certain, albeit peripheral interest here, in so far as none of them describe booster configurations of the kind discussed above:
U.S. Pat. No. 4,959,766 relates to an AC/DC converter having a tuned circuit at the input. At the beginning of the patent specification it is stated that switch mode represents an uninteresting solution in that connection. The solution described comprises thyristor control whereby a larger or smaller portion of the alternating voltage half period is utilized. The starting point is also different from that of the present invention, since signal transfer is not included, nor does any feeding at a constant current take place. PA0 U.S. Pat. No. 4,698,740 describes an AC/DC converter which is not based on a supply at a constant current either. However, there is described a solution with switch mode, so that this point in the patent specification has a certain similarity to the present invention. PA0 U.S. Pat. No. 3,906,337 relates to a DC/DC converter with a conventional rectifier bridge, i.e. rather remote from the basis of the present invention, as discussed above.